A systematic study on the photon localization of photonic-crystals (PCs) was conducted with respect to its PCs periodicity and its spatial distribution using the photo-degradation of acetaldehyde on nc-TiO2/SiO2 PCs composite films as the probe reaction. The periodicity of SiO2 PCs layer was tuned by the concentration of silica suspension in vertical deposition. The spatial distribution of photon localization was studied by inserting a SiO2 spacer with various thicknesses between the SiO2 PCs and nanocrystalline TiO2 (nc-TiO2). It was shown that the photocatalytic enhancement factor increased with the SiO2 PCs periodicity, indicating that, the more the periodicity of the PCs, the stronger the effect of its photon localization. However, when the PCs periodicity was more than 35, the enhancement factor tends asymptotically to a stationary value. In addition, the photocatalytic activity decreased with the gap distance between the nc-TiO2 catalytic layer and the PCs layer (i.e. the thickness of the SiO2 spacer), and when the gap distance is larger than 2.91.1,m, the PCs layer cannot enhance the photoactiyity of the nc-TiO2 any longer. The effect of the photon localization of PCs can exist outside the PCs, and weakens with the distance away from the surface of the PCs. This work can offer guidance for the fabrication and application of PCs-based photocatalysts. (C) 2015 Elsevier B.V. All rights reserved.